<i>In vitro</i> ruminal fermentation of leaves from three tree forages in response to incremental levels of polyethylene glycol

Abstract

Polyethylene glycol (PEG), a phenol binding agent has been used extensively to measure the biological activity of tannins in forage species. The optimum inclusion rate of PEG, per unit weight of sample varies from species to species. Determining optimum inclusion levels can prevent wastage and reduce the cost of diagnosing the biological activity of tannins, especially in developing countries. This study was designed to determine the optimum PEG inclusion levels required to completely ameliorate in vitro ruminal bioactivity of tannins in leaves from Leucaena leucocephala, Gliricidia sepium and Trichanthera gigantea using the Reading Pressure Technique. Fermentation parameters were generated by fitting gas production data to the Orskov and on-linear equation: . An asymptotic response to incremental levels of PEG was observed with cumulative gas production at 48 h post inoculation. The minimum level of PEG required to maximize in vitro ruminal fermentation of tree leaves was found to be 200 mg PEG/g DM for all tree species. Gas production rate constant for the insoluble fraction (c) showed an increase (P < 0.05) upon PEG addition for all species. In vitro organic matter degradability (iOMD) declined (P < 0.05) in the leaves of G. sepium (565 -540 g/kg DM) whereas, there was an increase (P < 0.05) in the iOMD of T. gigantea leaves (328 -340 g/kg DM) upon PEG addition. Partitioning factor (PF) declined (P < 0.05) upon PEG addition for all species. Predicted metabolizable energy was highest (P < 0.05) in the leaves of G. sepium (8.7 MJ/kg DM) and lowest in T. gigantea leaves (5.4 MJ/kg DM) upon PEG addition. It is concluded that a PEG inclusion level of 200 mg/g DM sample is sufficient for the diagnosis of in vitro ruminal tannin biological activity in leaves of the three tree species.